1. Field of the Invention
The present invention relates to a switching mode power supply (SMPS). More specifically, the present invention relates to a switching mode power supply for performing a protection operation.
2. Description of the Related Art
An SMPS converts a DC voltage into at least one other DC voltage. The DC output voltage can be controlled to be greater or less than the input voltage. The SMPS is applied to power electronic devices, in particular, battery power supply devices such as mobile phones and laptop computers.
In general, the SMPS has a protection circuit to protect the system and to maintain the security thereof against protection operation conditions such as overloads, short circuits, and open loops. When the SMPS has no protection circuit, elements may be damaged because of electrical stress. Therefore, the effective operation of the protection circuit has become increasingly important.
The general SMPS controls the output voltage by sensing the output voltage, receiving information regarding the sensed output voltage at a feedback terminal of a PWM controller, and controlling the turn-on time of a main switch according to the voltage of the feedback terminal. When the above-noted protection operation condition occurs, the voltage at the feedback terminal is saturated. U.S. Pat. No. 5,687,051, which is hereby incorporated in its entirety by reference, discloses a protection circuit of an SMPS in which the voltage at the feedback terminal is saturated under the protection operation condition. In this system, two current sources with different current levels are used to distinguish a long protection operation condition (which indicates that the protection circuit operation condition is maintained) and a short protection operation condition (which indicates that the protection circuit operation condition is not maintained). The protection circuit is controlled to be operable in the case of the long protection operation condition.
However, the system of the U.S. Pat. No. 5,687,051 has a feedback terminal for receiving information on the output voltage from the PWM controller and an additional terminal for sensing and receiving information on the current that flows through the main switch. The use of the additional terminal increases the number of elements which increases complexity and cost.